and am trying to treat with Maracyn-Oxy. I have a 10g tank (2 female bettas and 2 corys) with heater + filter (I cut out the carbon insert and am running it without this), and am on my third day of treatment. Sadly, last night one of my betta's head was super bloated and it's normal purple-blue color changed to a very patchy white and dull purple (almost grey) within hours, and then she died - all of this happened since adding Maracyn-Oxy for a white growth on its head. Now my other female betta looks terrible - white streak along side and very inactive. My cories seem unaffected.

I'm new to fish diseases - is Maracyn-Oxy potentially doing this to my bettas? Is there another better all-purpose treatment?

Corys are sensitive fish, so I don't think the meds killed the fish in and of itself. It could be that the fish just passed the threshold of being able to handle the added stress of the meds. It could have just been too little too late. We generally use meds as a last resort, which means the fish either hasn't responded to other treatments or has gotten worse. So, it's important to begin treatment as soon as possible, that way you have time to switch to meds when treatments don't work.

When things are bad, there is always the risk that the meds will kill the fish. However, death is certain if you don't use meds so there's really not much that can be done about that. You just have to do the best you can and hope for the best.